1. Field of the Invention
The present invention relates generally to kinematic arrangements for mounting strain sensitive devices such as optical elements and, more particularly, to an improved kinematic mounting arrangement with strain isolation, mechanical stiffness and distributed load capacity.
2. Description of Related Art
As described in U.S. Pat. No. 4,268,123 patented 19 May 1981, entitled "Kinematic Mount" and issued to the same assignee as herein, any movement of optical elements, when exposed to thermal expansion of their support, must be prevented to avoid optical distortion. For example, a lens, mirror, prism, or IR sensor mounted in a close fitting metal cell will maintain its relative alignment with respect to the cell in the absence of ambient temperature variations. However, problems may arise when such variations occur. Upon a decrease in temperature, the metal cell, where the coefficient of thermal expansion is normally higher than that of glass, will contract more rapidly than the glass, and press thereon to fracture the glass or at least stress the glass to the point of distorting the optical image which is transmitted through the glass or reflected therefrom. Conversely, as the temperature increases, the cell will expand more rapidly than the glass, leaving the lens somewhat unrestrained and free to move laterally within the cell, causing the optical element to become out-of-focus.
Various attempts have been proposed and used to avoid damage to an optical element, while still restraining it sufficiently to prevent excessive lateral displacement within its cell. For example, the element is encircled with a thin endless metallic strip which is pressed into place around the element and attached to the cell. However, where wide temperature variations are encountered, this method is objectionable because an excessive radial load must be applied by the strip to the element to hold the strip in place. Also, the metallic ring and optical element must have very accurate dimensions to fit the ring properly into the optical element and, in practice, it is usually necessary to grind the element to the encircling ring, a costly and time consuming practice.
In another commonly used mounting method, spring loaded members are equally spaced around the periphery of the element and positioned between it and its supporting cell. One disadvantage of this method is the difficulty of positively aligning the element in the cell. Another disadvantage is that the spring constants of the loading springs are or become unequal, especially after being subjected to a series of wide temperature variations. The result is a relative misalignment of the elements after extended use of the instrument in which the optical element is mounted.
A further method is to interpose some resilient material between a lens and its retaining cell to absorb lateral shock, and to compensate for the effects of temperature variation and attendant differential thermal expansion of the lens and the cell. The resilient material usually is a gasket or a series of equally spaced shims for effectively centering the element. One major objection to this method is that the interposed material deteriorates and loses its resiliency after the passage of time or after the optical system has been subjected to a series of wide temperature variations. Such deterioration of the interposed material may also result in a loss of relative alignment of the optical axis of the various optical elements in the system.
To overcome some of the disadvantages of the last-described method, tangent straps are placed around the optical element. These straps provide relatively low elastic joints which are useful for minimizing thermal expansion strains while allowing the optical element to remain fixed. The major objection to this method is that the thermal expansion strains are not entirely relieved due to the low elasticity of the tangent straps, thus causing the transmission through the optical element to become distorted. Additionally, the straps do not compensate for loading that cause the optical element to move out-of-plane.
The kinematic mount described in the above-referenced U.S. Pat. No. 4,268,123 overcomes the above problems by providing a novel ball-in-cylinder kinematic mounting arrangement for such an optical element by maintaining it in a relatively strain-free condition under a wide variety of temperature and mechanically induced motions. Specifically, the optical element is mounted and aligned relative to its housing by three mounts placed 120.degree. apart. Each mount includes a disc member having a spherically contoured peripheral surface and flat side surfaces, one of which is affixed to the edge of the optical element. The spherical surface is mounted in a cylindrical retaining sleeve and has an essentially one-dimensional contact therewith along a circular path. The resultant forces, exerted by the housing and the optical element on the three disc members, stabilize the disc members, and thereby create equilibrium in the mounting and aligning device.
Use of the above-described ball-in-cylinder kinematic mount has been found to be very effective for small, light optical elements and other components in which the circular contact between the ball and the cylinder supports the loads exerted therein. However, if the exerted load is too great at a point on the circle, the point contact can produce high localized stresses and permanent deformation, resulting from Brinelling, to degrade seriously the performance of the kinematic mount.